1. Field of the Invention
The present invention relates to an apparatus and a method for increasing semiconductor device density. In particular, the present invention relates to a method for producing vertically superimposed multi-chip devices usable with combined flip-chip, wire bond, and/or tape automated bonding (xe2x80x9cTABxe2x80x9d) assembly techniques to achieve densely packaged semiconductor devices.
2. State of the Art
Definitions: The following terms and acronyms will be used throughout the application and are defined as follows:
BGAxe2x80x94Ball Grid Array: An array of minute solder balls disposed on an attachment surface of a semiconductor die wherein the solder balls are refluxed for simultaneous attachment and electrical communication of the semiconductor dice to a printed circuit board. A BGA may also employ conductive polymer balls.
COBxe2x80x94Chip On Board: The techniques used to attach semiconductor dice to a printed circuit board, including flip-chip attachment, wirebonding, and TAB.
Flip-chip: A chip or die that has a pattern or array of terminations spaced around the active surface of the die for face down mounting of the die to a substrate.
Flip-chip Attachment: A method of attaching a semiconductor die to a substrate in which the die is inverted so that the connecting conductor pads on the face of the device are set on mirror-image pads on the substrate (such as a printed circuit board), and bonded by solder reflux or a conductive polymer curing.
Glob Top: A glob of encapsulant material (usually epoxy or silicone or a combination thereof) surrounding a semiconductor die in a COB assembly.
PGAxe2x80x94Pin Grid Array: An array of small pins extending substantially perpendicular from the major plane of a semiconductor die, wherein the pins conform to a specific arrangement on a printed circuit board or other substrate for attachment thereto.
SLICCxe2x80x94Slightly Larger than Integrated Circuit Carrier: An array of minute solder balls disposed on an attachment surface of a semiconductor die similar to a BGA, but having a smaller solder ball pitch and diameter than a BGA.
TABxe2x80x94Tape Automated Bonding. Conductive traces are formed on a dielectric film such as a polyimide (the structure also being termed a xe2x80x9cflex circuitxe2x80x9d), and the film is precisely placed to electrically connect a die and a circuit board or leadframe through the traces. Multiple connections are simultaneously effected.
State-of-the-art COB technology generally consists of three semiconductor die-to-printed circuit board conductive attachment techniques: flip-chip attachment, wirebonding, and TAB.
Flip-chip attachment consists of attaching a semiconductor die, generally having a BGA, a SLICC or a PGA, usually to a printed circuit board, although flip-chip attachment to leadframes is also known. With the BGA or SLICC, the solder or other conductive ball arrangement on the semiconductor die must be a mirror-image of the connecting bond pads on the printed circuit board such that a precise connection is made. The semiconductor die is bonded to the printed circuit board such as by refluxing the solder balls or curing the conductive polymer. With the PGA, the pin arrangement of the semiconductor die must be a mirror-image of the pin recesses on the printed circuit board. After insertion, the semiconductor die is generally bonded by soldering the pins into place. An under-fill encapsulant is generally disposed between the semiconductor die and the printed circuit board for environmental protection and to enhance the attachment of the die to the board. A variation of the pin-in-recess PGA is a J-lead PGA, wherein the loops of the Js are soldered to pads on the surface of the circuit board.
Wirebonding and TAB attachment generally begins with attaching a semiconductor die, usually by its back side, to the surface of a printed circuit board with an appropriate adhesive, such as an epoxy. In wirebonding, a plurality of bond wires are attached, one at a time, to each bond pad on the semiconductor die and extend to a corresponding lead or trace end on the printed circuit board. The bond wires are generally attached through one of three industry-standard wirebonding techniques: ultrasonic bondingxe2x80x94using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bondingxe2x80x94using a combination of pressure and elevated temperature to form a weld; and thermosonic bondingxe2x80x94using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. The die may be oriented either face up or face down (with its active surface and bond pads either up or down with respect to the circuit board) for wire bonding, although face up orientation is more common. With TAB, ends of metal leads carried on an insulating tape such as a polyimide are attached to the bond pads on the semiconductor die and to corresponding lead or trace ends on the printed circuit board. An encapsulant is generally used to cover the bond wires and metal tape leads to prevent contamination; TAB assemblies may be similarly encapsulated.
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. Greater integrated circuit density is primarily limited by the space or xe2x80x9creal estatexe2x80x9d available for mounting dice on a substrate such as a printed circuit board. Conventional leadframe design inherently limits package density for a given die size because the die-attach paddle of the leadframe must be larger than the die to which it is bonded. The larger the die, the less space that remains around the periphery of the die-bonding pad for wire bonding. Furthermore, the wire bonding pads on the standard leadframe provide anchorage for the leads when the leads and the die are encapsulated in plastic. Therefore, as the die size is increased in relation to a given package size, there is a corresponding reduction in the lateral depth along the sides of the package for the encapsulating plastic which joins the top and bottom of the plastic body at the mold part line and anchors the leads. Thus, as the leads and encapsulant are subjected to the normal stresses of subsequent trimming, forming and assembly operations, the encapsulating plastic may crack, compromising package integrity and substantially increasing the probability of premature device failure.
A so-called xe2x80x9cleads over chipxe2x80x9d (LOC) arrangement eliminates the die-attach paddle of the leadframe and supports the die by its active surface from the inner lead ends of the leadframe. This permits a wider variety of bond pad patterns on the die, extends the leads-to-encapsulant bond area and, with appropriate design parameters, can reduce the size of the packaged device for a given die size.
One method of increasing integrated circuit density is to stack dice vertically. U.S. Pat. No. 5,012,323 issued Apr. 30, 1991 to Farnworth teaches combining a pair of dice mounted on opposing sides of a leadframe. An upper, smaller die is back-bonded to the upper surface of the leads of the leadframe via a first adhesively coated, insulated film layer. A lower, larger die is face-bonded to the lower leadframe die-bonding region via a second, adhesively coated, insulative film layer. The wire-bonding pads on both upper and lower dice are interconnected with the ends of their associated lead extensions with gold or aluminum bond wires. The lower die must be slightly larger than the upper die so that the die pads are accessible from above through a bonding window in the leadframe such that gold wire connections can be made to the lead extensions. This arrangement has a major disadvantage from a production standpoint, since the different size dice require that different equipment produce the different dice or that the same equipment be switched over in different production runs to produce the different dice.
U.S. Pat. No. 5,229,647 issued Jul. 20, 1993 to Gnadinger teaches stacking wafers and using nonmechanically bonded electrical connections effected by metal-filled through holes contacting aligned conductive bumps of an adjacent wafer.
U.S. Pat. No. 5,291,061 issued Mar. 1, 1994 to Ball teaches a multiple stacked die device containing up to four stacked dice supported on a die-attach paddle of a leadframe, the assembly not exceeding the height of current single die packages, and wherein the bond pads of each die are wirebonded to lead fingers. The low profile of the device is achieved by close-tolerance stacking which is made possible by a low-loop-profile wirebonding operation and thin adhesive layers between the stacked dice.
U.S. Pat. No. 5,323,060 issued Jun. 21, 1994 to Fogal et al. teaches a multi-chip module that contains stacked die devices, the terminals or bond pads of which are wirebonded to a substrate or to adjacent die devices.
U.S. Pat. No. 5,422,435 to Takiar et al. teaches stacked dice having wire bonds extending to each other and to the leads of a carrier member such as a leadframe.
U.S. Pat. No. 5,399,898 issued May 21, 1995 to Rostoker teaches multi-chip, multi-tier semiconductor arrangements based on single and double-sided flip-chips. Using these dice to form a stacked die package eliminates the need for wirebonding and thus reduces the size of the stacked die package. However, these die stacks require double-sided flip-chips which are expensive and difficult to manufacture.
See also U.S. Pat. Nos. 5,146,308; 5,252,857; and 5,266,833 for additional background regarding die configurations and assemblies employing dice.
Therefore, it would be advantageous to develop a technique and assembly for increasing integrated circuit density using noncustomized die configurations in combination with commercially-available, widely-practiced semiconductor device fabrication techniques.
The present invention relates to a method for increasing integrated circuit density. The method comprises providing a pair of semiconductor wafers which are each provided on a face side with a desired integrated circuit pattern by techniques known in the industry. The completed wafers are then mounted back-to-back by a layer of adhesive material such that the back or blank sides face one another and the circuit-carrying or active sides face outward. The layer of adhesive, in some instances an electrically insulative adhesive, as required or desired to electrically isolate back-to-back adjacent dice, is applied over the back side of one wafer. The back side of the other wafer is placed on the adhesive, thereby attaching the wafers back-to-back to form a two-wafer stack. Depending on the adhesive, the assembly may be cured (as in the case of a thermosetting adhesive) to affix the wafers together. It is, of course, understood that the other types of adhesives not requiring a cure, such as pressure sensitive adhesives, could be used to hold the wafers back-to-back.
Generally, the wafers define identical die sizes, shapes and locations to facilitate singulation after assembly, and may include identical circuitry and bond pad arrangements on their respective dice, although this is not required. One technique for precisely aligning the wafers is to employ the wafer flats. It is, of course, understood that the more complex techniques such as pattern recognition systems could be employed to align the wafers through recognition of surface features and markings.
Once the wafers are aligned and adhered, the wafer stack is mounted onto a stretchable adhesive film carried by a frame, as known in the art. The adhesive film is used to immobilize the wafer stack for scribing or sawing to singulate the dice or excise portions or segments of the wafers, sawing being highly preferred over scribing due to the double thickness of the wafer stack. The wafer stack/film frame assembly is then sent through a dicing or singulation procedure wherein individual stacked dice pairs or wafer portions containing groups of dice pairs are cut from the wafer stack using a wafer dicing saw. The xe2x80x9cstreetsxe2x80x9d of the front and back wafers of the stack are, of course, aligned such that the circuitry on both wafers is not damaged during the dicing process.
It is, of course, understood that the above alignment, adherence and separation process can be employed using portions of wafers rather than full wafers, although this is not preferred and may be impractical since handling and sawing equipment is generally designed to accommodate full wafers, and proceeding at less than wafer level may be economically undesirable.
After the paired dice or wafer portions are removed from the wafer stack and adhesive film frame by a pick-and-place apparatus as known in the art, the film being stretched to enhance separation of component parts of the wafer at the sawing, they may be attached to a carrier substrate, such as a printed circuit board (PCB) or leadframe. A lower die of the dice pair is preferably configured as a flip-chip having, for example, BGA or SLICC solder bump connections, conductive polymer bumps, pin connections (PGA), or surface mount J-lead connections extending substantially perpendicular from the circuitry face of the lower die for attachment and electrical communication of the die to the carrier substrate. The substrate is configured with a specific lead end or trace end pattern compatible with the specific pin out or bump connections on the lower die.
An upper die of the dice pair can be used to align the dice pair to the substrate. The use of the upper die for alignment allows the dice pair to be placed within plus or minus 0.0002-0.0003 inch of the desired location. Optical alignment systems including, without limitation, pattern recognition systems as known in the art, are suitable to effect alignment to such tolerances. Once the dice pair is attached, an under-fill encapsulant is generally disposed between the lower die and the substrate (if other than a leadframe) for environmental protection and to enhance the attachment of the dice pair to the substrate.
Normally, the circuitry side or active surface of the upper die includes a plurality of bond pads. After attachment of the dice pair to the substrate, the bond pads of the upper die are brought into electrical communication with conductors of the substrate with wire bonds or TAB attachment. Bond wires and TAB traces of gold, aluminum or other suitable materials as known in the art are attached between the upper die bond pads and corresponding trace ends or lead ends of the substrate. If the die stack resides on a solid substrate such as a PCB, an encapsulant such as a glob-top is generally used to cover the bond wires or flex circuit to prevent contamination. If the die pair is secured to a leadframe, the assembly may be plastic encapsulated as known in the art, as by transfer molding. Of course, preformed ceramic or metal packaging as known in the art may also be employed.
It is, of course, also understood that both the upper and lower dice may be flip-chips having an array of minute solder balls or small pins for respective attachment to two opposing substrates such as facing PCBs or leadframes, or to a two-piece leadframe as known in the art.
The method of the invention increases semiconductor device density using noncustomized die and bond pad patterns and commercially-practiced mechanical and electrical attachment techniques.
It will also be recognized by those skilled in the art that the stacked wafers may be employed as a wafer level assembly, as in the case where extremely large memory capacity is required.
It will also be appreciated that, while the wafers have been described as adhered back-to-back, face-to-back adherence is also possible, that more than two wafers may be adhered with the use of appropriately-placed bond pads and that chamfering, undercutting or notching of the upper wafer edges may be effected to access lower wafer peripheral bond pads.